1. Field of the Invention
The present invention relates to a dielectric filter and a dielectric duplexer in which conductive through holes are provided in a dielectric block and in which an external conductor is provided on exterior surfaces of the dielectric block. The present invention also relates to a communication device using the dielectric filter and the dielectric duplexer.
2. Description of the Related Art
A typical dielectric filter is described with reference to FIGS. 11A and 11B. FIG. 11A is a perspective view of the dielectric filter and FIG. 11B is a front plan view of an open circuited end of the dielectric filter.
In FIGS. 11A and 11B, a dielectric block 1, through holes 2a to 2c with internal conductors 3a to 3c, an external conductor 4, conductor-free portions 5, input-output electrodes 6, and internal-conductor-free portions 7a to 7c are shown.
Preferably, the dielectric block 1 is in the form of a substantially rectangular solid. The holes 2a to 2c pass through the dielectric block 1 from one surface 1a to the opposite surface 1b. On the inside surface of the conductive through holes 2a to 2c, the internal conductors 3a to 3c are formed, respectively, so as to form respective conductive through holes. The external conductor 4 is preferably formed substantially on the whole outside surface of the dielectric block 1. The internal-conductor-free portions 7a to 7c are provided on the inside surface of the conductive through holes 2a to 2c such that the internal conductors 3a to 3c are separated from the external conductor 4 and form open circuited ends. In other words, the conductor-free portions 7a to 7c of each conductive through hole capacitively couple the conductive through holes to the external conductor and form the open circuited ends thereof. The other ends of the conductive through holes are directly coupled to the external conductor 4 so as to form the short circuited ends. In this way, dielectric resonators are formed by the internal conductors 3a to 3c, the dielectric block 1, and the external conductor 4.
On the outside surface of the dielectric block 1, the input-output electrodes 6 are formed so as to extend from opposite end faces of the dielectric block 1. The input-output electrodes 6 are preferably provided at opposite sides of the arrangement of the conductive through holes and are separated from the external conductor 4 by the external-conductor-free portions 5.
In this way, a dielectric filter is formed by the input-output electrodes 6 and the three dielectric resonators.
However, there are the following problems in such a dielectric filter which are illustrated with reference to FIGS. 12A to 12C. FIG. 12A is an equivalent circuit diagram of a two-stage dielectric resonator, FIG. 12B shows the state of electric lines of force in even mode and in odd mode, and FIG. 12C is an equivalent circuit diagram of a two-stage dielectric resonator having a jumping coupling capacitance.
In an integral type dielectric filter composed of a plurality of resonators using a dielectric block, tip capacitance Cs is generated between an open end of the resonator and the external conductor as a grounding electrode shown in FIG. 12A.
The electric lines of force where the tip capacitance Cs is generated in even mode and in odd mode are shown in FIG. 12B. In even mode, the electric lines of force are generated between the resonators and the grounding electrode. In odd mode, a part of the electric lines of force is generated between the resonators. Therefore, the tip capacitance Cs generated between the resonators and the grounding electrode in odd mode becomes smaller than that in even mode, and jumping tip capacitance dCs is generated between the open ends of the resonators. Here, since Cs is set on the basis of the capacitance in even mode, the jumping coupling capacitance dCs has a minus value.
In this way, when the jumping coupling capacitance dCs generated between the open ends of the resonators is considered, the equivalent circuit diagram shown in FIG. 12A becomes the circuit diagram in FIG. 12C.
A three-stage dielectric resonator is described with reference to FIGS. 13A and 13B. FIG. 13A is an equivalent circuit diagram of the three-stage dielectric resonator and FIG. 13B shows the attenuation characteristics of a dielectric filter provided with the three-stage dielectric resonator.
As shown in FIG. 13A, the tip capacitance Cs is generated between the open end and the external conductor as the grounding electrode in each resonator, and jumping coupling capacitance dCs1 is generated between the open ends of neighboring resonators, respectively. Furthermore, jumping coupling capacitance dCs2, which is very small compared to the jumping coupling capacitance dCs1 generated between the open ends of neighboring resonators, is also generated between the open ends of the non-neighboring resonators at both ends of the array of resonators.
Here, since the jumping coupling capacitance dCs1 generated between neighboring resonators is included in the coupling capacitance between resonators, the capacitance does not have great effects on the attenuation characteristics, but, since the jumping coupling capacitance Cs2 generated between the non-neighboring resonators is different from the coupling capacitance between resonators, the capacitance has an effect on the position of the attenuation poles as shown in FIG. 13B. For example, in a dielectric filter composed of a three-stage resonator in which they have combined (inductive) coupling, two attenuation poles are created on the higher-frequency side of the passband If the jumping coupling capacitance dCs2 is large, the space between the attenuation poles increases and, if the jumping coupling capacitance dCs2 is small, the space between the attenuation poles decreases. Therefore, desired attenuation characteristics cannot be obtained outside the passband, although they are dependent on the position where the attenuation poles are generated.
In order to solve this problem, dielectric filters shown in FIGS. 14A and 14B have been used.
FIGS. 14A and 14b are perspective views of dielectric filters.
In the dielectric filter shown in FIG. 14A, the inner diameter of the conductive through hole 2b is larger than those of the other conductive through holes 2a and 2c. In the dielectric filter shown in FIG. 14B, the inner diameter of the conductive through hole 2b is smaller than those of the other conductive through holes 2a and 2c. 
In the dielectric filter shown in FIG. 14A, since the inner diameter of the conductive through hole 2b is large, the space between the internal conductor 3b and the external conductor 4 becomes smaller and the jumping coupling capacitance dCs2 generated between the internal conductor 3a and the internal conductor 3c decreases. Since the inner diameter of the conductive through hole 2b is not appropriate for obtaining the optimum Q0, Q0 of the resonators becomes smaller and adverse effects are added, such as insertion loss.
In the dielectric filter shown in FIG. 14B, since the inner diameter of the conductive through hole 2b is small, the space between the internal conductor 3b band the external conductor 4 becomes larger and the jumping coupling capacitance dCs2 generated between the internal conductor 3a and the internal conductor 3c increases. Since the inner diameter of the conductive through hole 2b is not appropriate for obtaining the optimum Q0, Q0 of the resonators also becomes smaller in this case and adverse effects are produced, such as insertion loss.
Accordingly, it is an object of the present invention to provide a dielectric filter and dielectric duplexer in which the deterioration of Q0 of resonators is suppressed, jumping coupling capacitance generated between non-neighboring resonators is controlled, attenuation poles are established at desired locations, and the attenuation characteristics are improved outside the passband. It is also an object to provide a communication device having the dielectric filter or the dielectric duplexer of the present invention.
In accordance with a first embodiment of the present invention, a dielectric filter includes a dielectric block having first and second opposed surfaces, the first and second opposed surfaces having a width direction and a length direction greater than the width direction. An external conductor is formed on exterior surfaces of the dielectric block and at least three conductive through holes arrayed in the length direction extend from the first to the second surface of the dielectric block. Each conductive through hole has a short circuit end directly coupled to the external conductor and an open circuit end capacitively coupled to the external conductor. A sectional shape of at least one conductive through hole located between two other conductive through holes of the at least three conductive through holes is elongated in the width direction. With this, capacitance generated between the conductive through holes on both sides of the at least one conductive through hole is reduced, and attenuation pole frequencies are shifted so that the space between two attenuation poles due to the jumping coupling between the resonators of the two non-neighboring conductive through holes may be narrowed.
In a second embodiment, the dielectric filter includes a dielectric block having first and second opposed surfaces, the first and second opposed surfaces having a width direction and a length direction greater than the width direction. An external conductor is formed on exterior surfaces of the dielectric block and at least three conductive through holes arrayed in the length direction extend from the first to the second surface of the dielectric block. Each conductive through hole has a short circuit end directly coupled to the external conductor and an open circuit end capacitively coupled to the external conductor. A sectional shape of two conductive through holes on either side of a third conductive through hole of the at least three conductive through holes is elongated in the width direction. With this, capacitance generated between the two elongated conductive through holes is increased, and attenuation pole frequencies are shifted so that the space between two attenuation poles due to the jumping coupling between the resonators of the two non-neighboring conductive through holes may be widened.
In a further embodiment of the present invention, the dielectric filter is constructed such that the cross-sectional shape of all of the conductive through holes is elongated in the width direction of the dielectric block.
In another embodiment, the dielectric filter of the present invention is constructed such that the conductive through holes are stepped holes in which the inner diameter on the open circuited end is different from the inner diameter on the short-circuited end. It is preferred that the stepped through hole is the elongated through hole.
In still a further embodiment, the dielectric filter of the present invention is constructed such that the axial position of the stepped conductive through holes on the open circuited end is different from the axial position on the short circuited end.
In one aspect of the present invention, the above dielectric filter is used in a dielectric duplexer. In another aspect of the present invention, a communication device is formed using the above dielectric filter or the above dielectric duplexer.
The term xe2x80x9ccross sectionxe2x80x9d refers to a section of the conductive through holes taken perpendicular to the axial direction of the holes. Hereinafter, the cross-sectional shape of the internal conductors is referred to as the sectional shape.